1. Field of the Invention
This invention relates to a diffractive optical element and a method of manufacturing the same, and particularly to a diffractive optical element for use in light of a plurality of wavelengths or bands and a method of manufacturing the same.
2. Related Background Art
In a refracting optical system, use has heretofore been made of a method of decreasing chromatic aberration by combining glass materials differing in dispersion. In contrast, a method of decreasing chromatic aberration by providing a diffractive optical element having the diffracting action on a lens surface or in a portion of an optical system is disclosed in such literature as SPIE, Vol. 1354, International Lens Design Conference (1990), Japanese Patent Application Laid-Open No. 4-213421 (corresponding U.S. Pat. No. 5,0447,06), Japanese Patent Application Laid-Open No. 6-324262 (corresponding U.S. Pat. No. 5,790,321), etc. These correct chromatic aberration by the utilization of the phenomenon that chromatic aberration appears in opposite directions on a refractive part and a diffractive part disposed in an optical system. Also, such a diffractive optical element is comprised of phase-type diffractive gratings having synchronism, and can also be given an effect like that of an aspherical lens by varying the period of the periodic structure thereof, and is greatly effective to reduce aberrations.
The diffractive optical element having the above-described phase-type diffractive gratings, unlike a conventional lens optically acting by the refractive index of the medium thereof and the profile of the surface thereof, performs action similar to that of a lens which causes incident light to converge or diverge by a diffractive phenomenon occurring due to the synchronism of the diffractive gratings. The shape of the diffractive gratings in such a diffractive optical element has is a concentric circular shape centering around a point, and the diffractive grating nearest to the central point is called a first zone, and the subsequent diffractive gratings are called a second zone, a third zone, and so on.
Also, such a diffractive optical element can be made by injection molding using a resin molding metal mold, etching, laser beam machining or the like. Generally the injection method is adopted because it is advantageous during mass production. For the working of the metal mold used in this injection method, cutting using a diamond turning tool, i.e., the so-called diamond turning, is widely used.
The aligning when the diffractive optical element made by this method is mounted on a lens holder is presented in Japanese Patent Application Laid-Open No. 10-274705. The diffractive optical element has its grating structure determined so that a beam in the entire area of the wavelength used may concentrate in a particular order (hereinafter referred to also as the design order), and in addition, is designed such that the diffractive efficiency thereof becomes high at a certain wavelength of the design order (hereinafter referred to also as the design wavelength).
The diffractive efficiency at the particular diffractive order when a diffractive optical element as shown in FIG. 9 of the accompanying drawings is formed on a certain surface is shown in FIG. 10 of the accompanying drawings.
When use is made of a diffractive optical element in which the diffractive grating is constituted by only one layer like this, diffractive efficiency becomes highest at the design wavelength, and gradually becomes lower at the other wavelengths. These amounts of reduction in diffractive efficiency by the diffractive grating of a single layer become diffracted lights of the other orders, and become a cause of the flare on the image plane. Also, particularly when a plurality of diffractive optical elements are used, the reduction in diffractive efficiency at the other wavelengths than the design wavelength leads to a reduction in transmittance.
A construction which can decrease this reduction in diffractive efficiency is proposed in Japanese Patent Application Laid-Open No. 11-223717. According to this, high diffractive efficiency is maintained in the entire area of the wavelength used by a diffractive optical element of structure in which diffractive gratings of at least two layers are laminated. Flare or the like is also effectively restrained thereby.
When a diffractive optical element of such a laminated type is to be manufactured, the alignment of the diffractive gratings of respective layers becomes an important matter. It is because unless the diffractive gratings of the respective layers are accurately aligned, diffractive efficiency is reduced all the more in spite of being special laminated structure which can realize high diffractive efficiency over a wide wavelength band.
Regarding a method of aligning two diffractive gratings, for example, Japanese Patent Application Laid-Open No. 2000-114143 discloses a construction in which a concave portion and a convex portion are provided near the centers of the optical effective portions of the two diffractive gratings and they are fitted together to thereby effect positioning.
However, when the technique disclosed in the above-mentioned publication is adopted for a diffractive optical element in which two diffractive gratings are disposed with an air layer therebetween, the positioning convex portion becomes longer by an amount corresponding to the air layer and therefore, there is the following problem. If the convex portion is made thin so as not to affect optical performance, uncertainty of strength will occur, and if the convex portion is made thick so as not to pose a problem in strength, influence upon optical performance will be feared.